Process and apparatus for vacuum treatment of metals

ABSTRACT

A process and apparatus for the vacuum treatment of metals. In a tightly sealed vacuum treatment vessel, molten metal is preferably divided, on the surface, into a generally circular sector and a generally ring-shaped sector surrounding the generally circular sector. Preferably, each sector of the molten metal is exposed to different vacuum pressures during vacuum treatment, wherein a generally lower vacuum pressure is applied to the ring-shaped sector than to the circular sector. The two sectors are preferably separated to an immersion depth, in the molten bath, of between 10 and about 20 cm, as measured starting from the ring-shaped sector.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a process and an apparatus for the vacuumtreatment of metals, in particular steel.

2. Background Information

During ladle degassing, the tapping ladle filled with steel is loweredinto a large cylindrical chamber, and this chamber is then sealed vacuumtight by a cover. The seal is generally a rubber ring. The covers aremanufactured either of steel castings or of sheet metal structures. Onthe underside of the cover there is an ejector protector made of sheetmetal and/or refractory tamping clay. Alloy addition devices and viewingports are located on the cover.

The vacuum is generally generated by at least four ejectors. When moltensteel is exposed to a vacuum pressure, gas bubbles form inside the steelat a pressure which is a function of the internal pressure above thesurface of the bath. Unkilled steel with a high oxygen contentexperiences an agitation action as a result of the formation of carbonmonoxide even at pressures of less than 200 Torr in the open space ofthe vacuum treatment vessel, and thus simultaneously flushes thehydrogen and nitrogen out of the molten metal, so that even with thisrelatively weak vacuum a removal of gases occurs. As the pressure isfurther reduced, the agitation can become vigorous, so that for example,the molten steel can rise one meter and more in the ladle. Sufficientroom for the steel to rise must be provided by the selection of a ratherlarge ladle is necessary, and therefore the ladle cannot be filled toits edge, rather it must have a certain freeboard. In existing steelmills, the ladle sizes and weights when full are determined as afunction of the crane equipment. As a result of the need for thefreeboard, the ladles can no longer be filled to the rim, which has thedisadvantage of reducing production. The alternative solution, ofenlarging the ladles, means that the hoisting apparatus and the pick-upequipment must be adjusted to the increased weight to be transported.

An absorption vessel independent of the ladle offers an additionalsolution. DE-OS 20 32 830 discloses a plunger, which is immersed withthe open side down in the molten metal, and its inside is thenevacuated. This plunger has the disadvantage that it must be pushed intothe molten metal to achieve the required depth of immersion during thevacuum treatment. After the vacuum pressure is established, the level ofthe surface of the molten metal rises by the barometric differential,which can be much more than 1 meter, while the level of the molten metalnot affected by the vacuum drops by a similar amount. As a result of theenclosure of molten metal in the plunger, which is smaller than theladle, a relatively large volume of the molten metal is separated fromthe molten metal remaining in the ladle, with the disadvantage that thetwo portions of the molten metal are exposed to different vacuumtreatments.

DE-AS 19 65 136 discloses an apparatus for the ladle degassing of moltenmetals, in which a reaction tube located underneath the cover of thevacuum treatment vessel can be immersed in the molten metal. In acomplex manner, a lance with reactive gases is guided into the spaceenclosed by the reaction tube for the metallurgical treatment, duringwhich the degassing and thus the volume increase of the molten metal isto take place. On account of the uniform vacuum pressure acting on thesurface of the molten metal, it is not possible to prevent an increasein volume in the ring-shaped area between the reaction tube and the edgeof the ladle.

DE-AS 19 12 907 and 19 19 053 also disclose devices in which gas isintroduced into the molten metal through a tubular partition which isimmersed in the molten metal. This partition is surrounded in aring-shaped manner by an additional tubular partition, so that there isa communicating connection between these two partitions. By connectionto pressure and/or suction pumps at different pressures, differentsurface level heights can be obtained in the individual chambers, andultimately this leads to an improved flow of the metal or bath movement.

OBJECT OF THE INVENTION

The object of the invention is to find a process and an apparatus forthe vacuum treatment of metals, in particular steel, which eliminate theabove-mentioned disadvantages, use simple means, eliminate therequirement for a freeboard in the ladle and do not interfere with thedegassing of the melt.

This object is achieved by means of the invention disclosed hereinbelow.

SUMMARY OF THE INVENTION

According to the invention, a structurally simple, relatively small andlightweight skirt is located on the cover of the vacuum treatmentvessel. The diameter of this skirt is only slightly smaller than thediameter of the ladle at the level of immersion. The lower, open edge ofthe skirt is immersed only slightly in the molten metal. The immersedskirt divides the surface of the molten metal into two sectors, onering-shaped and one circular, to which different vacuum pressures areapplied.

The pressure difference can be adjusted as desired. The preferred rangeis between 1/2 and 2 pressure stages.

On account of the size of the shield, the radius of which is in a ratioof 8:1 to 122:1 to the width of the ring-shaped sector, there is hardlyany interference with the vacuum treatment of the entire amount of themolten metal. This effect is further improved by the fact that the depthof penetration of the skirt is limited to a minimum, and thereby assumesvalues of 10 to 20 cm. The three-dimensional areas mentioned above andthe low depth of immersion of the skirt do not significantly disrupt theflow conditions in the molten metal. This has a particularlyadvantageous effect with the high flow velocities of the molten metalused in modern installations, caused by the large amount of stirring gasused, which is introduced into the molten metal through up to threeporous plugs.

This effect can hardly be improved any further, if the shield isdesigned to be vertically adjustable, since depending on the level towhich the ladle is filled, the immersion depth of the skirt can beadjusted at any stage of the vacuum treatment.

The pressure difference can either be set directly between two pressurestages, or can be continuously adjusted by using a branch withthrottles.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings contain schematic illustrations of examples ofthe invention.

FIG. 1 shows a cross section through the vacuum treatment vessel with asolid cover, with connections to different stages of a vacuum generator.

FIG. 1a, like FIG. 1, with additional electrode in open space A,

FIG. 2 shows a vacuum treatment vessel with an axially movable skirt,

FIG. 3 shows the connection of the vacuum treatment vessel to the vacuumgenerator via a branch and throttle.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 1 and 1a show a vacuum treatment vessel 30 with a flange andgasket, on which a cover 20 is placed. In the vacuum treatment vessel30, there is a ladle 40 filled with molten metal 41. The lower edge 22of a skirt 21 fastened to the cover 20 is immersed in the molten metal41.

The skirt 21 immersed in the surface of the molten metal bath dividesthe latter into a circular segment 42 and a ring-shaped segment 43.

The open space A is enclosed between the circular bath surface 42, theinner jacket of the skirt 21 and the circular portion 23 of the cover20. The remaining portion of the cover 20, with the ring-shaped part 29,the jacket outside of the skirt 21, the lower portion of the vacuumtreatment vessel 30, the outside of the ladle 40 and the ring-shapedsurface 43 of the molten metal enclose the open space B.

For viewing the surface 42, 43 of the molten metal, there are viewingports 33, 34 in the cover 20. The open space A is connected to thevacuum system 10 by means of a connection 24 in the vicinity of thecircular cover 23, and the open space B is connected to the vacuumsystem 10 by means of a connection 25, in the vicinity of the annularportion 29 of the cover.

The vacuum generator 10 has a water ring pump 14, a steam ejector 13 (60Torr), and between the two a condenser 16. It also has a steam ejector12 (10 Torr) and a steam ejector 11 (0.5 (Torr), and between theejectors 12 and 13 there is a condenser 15. The open space A isconnected to the maximum vacuum pressure stage p1 of the steam ejector11, and the open space B in the present case is connected two stageslower p2, between the ejectors 13 and 12.

During the operation of the vacuum generator 10, the height of thecircular segment 42 of the molten metal bath surface rises by the amounta in relation to the ring-shaped surface 43.

FIG. 1a shows, in addition to the system described above in FIG. 1, anelectrode 60, which projects through the electrode duct 61 in thevicinity of the circular cover portion 23 into the open space A.

FIG. 2 is a schematic illustration of a vertically adjustable skirt 21,which is fastened to a circular portion 23 of the cover 20, whereby thecircular portion 23 can be adjusted by means of adjustment elements 51in relation to the ring-shaped portion 29 of the cover 20. For agas-tight seal there are compensators 53 between the ring-shaped portion29 and the circular portion 23. The connection 25 to the open space B islocated in one case in the cover 20, and in the other case in the lowerportion of the vessel of the vacuum treatment vessel 30.

FIG. 3 shows the essential elements of FIG. 1, with the distinction thatthere is a connection from the connection 24 to the open space A bymeans of a branch 26, which is simultaneously connected to theconnection 25 of the open space B, whereby there is a throttle 27between the branch 26 and the connection 25.

One aspect of the invention resides broadly in a process for the vacuumtreatment of metals, in particular steel, in which molten metal is in avacuum treatment vessel tightly sealed by the cover, whereby the surfaceof the molten metal is divided into a circular sector and a ring-shapedsector surrounding the latter, and the molten metal is exposed todifferent vacuum pressures on its surface during the vacuum treatment,characterized by the fact that a lower vacuum pressure is applied to thering-shaped sector than to the circular sector, and that the separationof the sectors is made to an immersion depth in the molten bath between10-20 cm measured, starting from the ring-shaped sector.

Another aspect of the invention resides broadly in a processcharacterized by the fact that a difference of the vacuum pressurebetween the ring-shaped and the circular sector is selected whichcorresponds to at least one-half of a pressure stage of the vacuumsystem.

Yet another aspect of the invention resides broadly in an apparatus forthe performance of a process with a vacuum treatment vessel which has acover equipped with a gasket, and which is connected to a multi-stagevacuum generation system, whereby a ladle filled with molten metal canbe introduced into the vacuum treatment vessel, characterized by thefact that the cover 20 has a cylindrical skirt 21 oriented parallel tothe center axis 1 of the cover 20, whose peripheral region 22 projectsinto the molten metal 41 when the ladle 40 is filled, and the diameterof which is only slightly less than the diameter of the ladle at theimmersion level, that there are connections 24, 25 between the vacuumsystem 10 and the open space A separated by the skirt 21 surrounded bythe circular portion 23 of the cover 20--when the ladle 40 is filled--bythe circular surface 42 of the molten metal, and with the open space B,which is surrounded by the ring-shaped portion 29 of the cover 20, thevacuum treatment vessel 30, the ladle 40 and--when the ladle isfilled--by the ring-shaped surface 43 of the molten metal, where theconnection 24 to the empty space A is connected on an ejector 11 havingthe highest pressure stage of the vacuum system 10, and the connection25 to the empty space B is made to the ejector 12 having at least thesecond-highest pressure stage of the vacuum system 10.

A further aspect of the invention resides broadly in an apparatuscharacterized by the fact that on the connection 25 to the open space A,there is a branch 26 equipped with throttle elements 27 and which isconnected to the open space B.

A yet further aspect of the invention resided broadly in an apparatuscharacterized by the fact that the cover 20 has a ring-shaped opening 28and that there are adjustment elements 51 on the cover 20, through whichthe skirt 21 can be moved parallel to the center axis 1.

Yet another further aspect of the invention resides broadly in anapparatus characterized by the fact that compensators 53 are fastenedgas-tight between the ring-shaped portion 29 of the cover 20 and theedge 23 of the skirt 21 pointing away from the ladle 4.

An additional aspect of the invention resides broadly in an apparatuscharacterized by the fact that at least one electrode 60 can beintroduced through the cover 20.

All, or substantially all, of the components and methods of the variousembodiments may be used with at least one embodiment or all of theembodiments, if any, described herein.

All of the patents, patent applications and publications recited herein,if any, are hereby incorporated by reference as if set forth in theirentirety herein.

The details in the patents, patent applications and publications may beconsidered to be incorporable, at applicant's option, into the claimsduring prosecution as further limitations in the claims to patentablydistinguish any amended claims from any applied prior art.

The appended drawings in their entirety, including all dimensions,proportions and/or shapes in at least one embodiment of the invention,are accurate and to scale and are hereby incorporated by reference intothis specification.

The invention as described hereinabove in the context of the preferredembodiments is not to be taken as limited to all of the provided detailsthereof, since modifications and variations thereof may be made withoutdeparting from the spirit and scope of the invention.

What is claimed is:
 1. Process for the vacuum treatment of steel, inwhich molten metal is in a vacuum treatment vessel tightly sealed by acover, whereby the surface of the molten metal is divided into acircular sector and a ring-shaped sector surrounding the latter, and themolten metal is exposed to different partial vacuums on its surfaceduring the vacuum treatment, wherein a lower partial vacuum is appliedto the ring-shaped sector than to the circular sector, and that theseparation of the sectors is made to an immersion depth in the moltenbath between 10-20 cm measured, starting from the ring-shaped sector. 2.Process according to claim 1, wherein a difference of the vacuumpressure between the ring-shaped and the circular sector is selectedwhich corresponds to at least one-half of a pressure stage of the vacuumsystem.
 3. Apparatus for the vacuum treatment of steel, said apparatuscomprising:a vacuum treatment vessel which has a cover equipped with agasket, and which is connected to a multi-stage vacuum generationsystem, whereby a ladle filled with molten metal can be introduced intothe vacuum treatment vessel, the cover has a cylindrical skirt orientedparallel to the center axis of the cover, whose peripheral regionprojects into the molten metal when the ladle is filled, and thediameter of which is only slightly less than the diameter of the ladleat the immersion level, wherein: there are connections (24, 25) betweenthe vacuum system (10) and:the open space (A) separated by the skirt(21) surrounded by the circular portion (23) of the cover (20)--when theladle (40) is filled--by the circular surface (42) of the molten metal,and with the open space (B), which is surrounded by the ring-shapedportion (29) of the cover (20), the vacuum treatment vessel (30), theladle (40) and--when the ladle is filled--by the ring-shaped surface(43) of the molten metal,where the connection (24) to the empty space(A) is connected on an ejector (11) providing the highest partial vacuumof the vacuum system (10), and the connection (25) to the empty space(B) is made to the ejector (12) on a section of the vacuum system (10)providing a smaller partial vacuum than on connection
 24. 4. Apparatusaccording to claim 3, wherein:on the connection (25) to the open space(A), there is a branch (26) equipped with throttle elements (27) andwhich is connected to the open space (B).
 5. Apparatus according toclaim 3, wherein:the cover (20) has a ring-shaped opening (28) and thatthere are adjustment elements (51) on the cover (20), through which theskirt (21) can be moved parallel to the center axis (1).
 6. Apparatusaccording to claim 5, wherein:compensators (53) are fastened gas-tightbetween the ring-shaped portion (29) of the cover (20) and thecircle-shaped part (23) of it in the shape of edge (23) of the skirt(21) pointing away from the ladle (40).
 7. Apparatus according to claim3, wherein:at least one electrode (60) can be introduced through thecover (20).
 8. Apparatus according to claim 4, wherein at least oneelectrode (60) can be introduced through the cover (20).
 9. Apparatusaccording to claim 5, wherein at least one electrode (60) can beintroduced through the cover (20).
 10. Apparatus according to claim 6,wherein:at least one electrode (60) can be introduced through the cover(20).
 11. Process according to claim 1, comprising the stepsof:providing a vacuum treatment vessel which has a cover equipped with agasket, providing a multi-stage vacuum generation system, connecting thevacuum treatment vessel to the multi-stage vacuum generation system,providing a ladle for being filled with molten metal, filling the ladlewith molten metal, introducing the ladle, filled with molten metal, intothe vacuum treatment vessel, configuring the cover to have a cylindricalskirt oriented parallel to the center axis of the cover, whoseperipheral region projects into the molten metal when the ladle isfilled, and the diameter of which is only slightly less than thediameter of the ladle at the immersion level, wherein said processfurther comprises the steps of: providing connections (24, 25) betweenthe vacuum system (10) and:an open space (A) separated by the skirt (21)surrounded by the circular portion (23) of the cover (20)--when theladle (40) is filled--by the circular surface (42) of the molten metal,and an open space (B), which is surrounded by the ring-shaped portion(29) of the cover (20), the vacuum treatment vessel (30), the ladle (40)and--when the ladle is filled--by the ring-shaped surface (43) of themolten metal, wherein the connection (24) to the empty space (A) isconnected on an ejector (11) providing the highest partial vacuum of thevacuum system (10), and the connection (25) to the empty space (B) ismade to the ejector (12) on a section of the vacuum system (10) with asmaller partial vacuum than on connection (24).
 12. Process according toclaim 11, further comprising:providing, on the connection (25) to theopen space (A), a branch (26); equipping the branch with throttleelements (27); and connecting the branch to the open space (B). 13.Process according to claim 11, further comprising:configuring the cover(20) to have a ring-shaped opening (28); providing adjustment elements(51) on the cover (20), by means of which adjustment elements the skirt(21) can be moved parallel to the center axis (1).
 14. Process accordingto claim 13, further comprising:providing compensators (53) for beingfastened gas-tight between the ring-shaped portion (29) of the cover(20) and the circle-shaped part (23) of it in the shape of edge (23) ofthe skirt (21) pointing away from the ladle (40).
 15. Process accordingto claim 14, further comprising the step of:providing at least oneelectrode (60) for being introduced through the cover (20).
 16. Processaccording to claim 11, further comprising:providing at least oneelectrode (60) for being introduced through the cover (20); andintroducing the at least one electrode through the cover.
 17. Processaccording to claim 12, further comprising:providing at least oneelectrode (60) for being introduced through the cover (20); andintroducing the at least one electrode through the cover.
 18. Processaccording to claim 13, further comprising:providing at least oneelectrode (60) for being introduced through the cover (20); andintroducing the at least one electrode through the cover.
 19. Method forthe vacuum treatment of molten metal, said method comprising the stepsof:providing a ladle for holding the molten metal; conducting the moltenmetal in the ladle, the molten metal having an upper surface whendisposed in the ladle; providing a vacuum treatment vessel; disposingthe ladle, with the molten metal, within the vacuum treatment vessel;sealing the vacuum treatment vessel; providing means for applying atleast one partial vacuum at the upper surface of the molten metal fordegassing the molten metal; providing means for dividing the uppersurface of the molten metal into a first portion and a second portion;configuring the dividing means such that the first portion of the uppersurface of the molten metal surrounds the second portion of the uppersurface of the molten metal; configuring the means for applying at leastone partial vacuum to apply a first partial vacuum to the first portionof the upper surface of the molten metal and a second, different,partial vacuum to the second portion of the upper surface of the moltenmetal; applying the first partial vacuum to the first portion of theupper surface of the molten metal and the second partial vacuum to thesecond portion of the upper surface of the molten metal; and immersingthe dividing means in the molten metal to a depth at which flowconditions within the molten metal are not significantly disrupted. 20.The method according to claim 19, wherein the ladle has a diameter at anupper portion thereof, said method further comprising the stepsof:immersing the dividing means in the molten metal to a depth ofbetween about 10 centimeters and about 20 centimeters with respect tothe second portion of the upper surface of the molten metal; configuringthe means for applying at least one partial vacuum such that, duringsaid step of applying the first partial vacuum and the second partialvacuum, the first partial vacuum is greater than the second partialvacuum, thereby resulting in a lower absolute pressure at the firstportion of the upper surface of the molten metal than at the secondportion of the upper surface of the molten metal; configuring thedividing means such that, when the dividing means is immersed in themolten metal, the first portion of the upper surface of the molten metalcompletely surrounds the second portion of the upper surface of themolten metal; providing a cover for covering the vacuum treatmentvessel; configuring the cover of the vacuum treatment vessel to comprisethe dividing means; configuring the dividing means to comprise skirtmeans for extending downwardly towards the molten metal, the skirt meansfor being immersed in the molten metal; configuring the skirt means tocomprise a generally cylindrical shape, the skirt means having a radiusand a diameter; configuring the dividing means such that, when thedividing means is immersed in the molten metal, the first portion of theupper surface of the molten metal is generally ring-shaped and thesecond portion of the upper surface of the molten metal is generallycircular in shape, the first portion of the upper surface of the moltenmetal having a width; configuring the dividing means such that, when thedividing means is immersed in the molten metal, the first portion of theupper surface of the molten metal has a surface area substantially lessthan that of the second portion of the upper surface of the moltenmetal; configuring the skirt means such that the diameter thereof isslightly less than the diameter of the ladle configuring the skirt meanssuch that, when the skirt means is immersed in the molten metal, theratio of the radius of the skirt means to the width of the first portionof the upper surface of the molten metal is between about 8:1 and about122:1; configuring the means for applying at least one partial vacuum toprovide a pressure difference, between the first partial vacuum and thesecond partial vacuum, of between about 1/2 and about 2 stages;providing, during said step of applying the first partial vacuum and thesecond partial vacuum, a pressure difference, between the first partialvacuum and the second partial vacuum, of between about 1/2 and about 2stages; providing means for vertically adjusting the skirt means, tovary the depth of immersion of the skirt means; vertically adjusting theskirt means to vary the depth of immersion of the skirt means;configuring the cover to comprise a first portion and a second portion,the first portion of the cover being generally ring-shaped and thesecond portion of the cover being generally circular in shape;configuring the cover such that the skirt means extends downwardly fromthe second portion of the cover; providing a plurality of compensatorsfor being fastened in a gas-tight manner between the first portion ofthe cover and the second portion of the cover to provide a gas-tightseal between the first portion of the cover and the second portion ofthe cover; providing at least one electrode for being introduced throughthe cover; introducing the at least one electrode through the cover.